The nucleus accumbens is found in the ventral striatum and is composed of medium spiny neurons.[3][4] It is subdivided into limbic and motor subregions known as the shell and core.[2] The medium spiny neurons receive input from both the dopaminergic neurons of the VTA and the glutamatergic neurons of the hippocampus, amygdala, and medial prefrontal cortex. When they are activated by the these inputs, the medium spiny neurons' projections release GABA onto the ventral pallidum.[2] The release of dopamine in this structure drives the mesolimbic system.

Amygdala

The amygdala is a large nuclear mass in the temporal lobes anterior to the hippocampus. It has been associated with the assignment of emotions, especially fear and anxiety.

Hippocampus

The hippocampus is located in the medial portion of the temporal lobes. It is known for its association with memory.

There is controversy about dopamine’s role in the reward system. Three hypotheses—hedonia, learning, and incentive salience—have been proposed as explanations for dopamine’s function in the reward system.[5] The hedonia hypothesis suggest that dopamine in the nucleus accumbens is a 'pleasure neurotransmitter'. Historically, in the late 1970s, it was found that drugs of abuse, worked with dopamine particularly in the nucleus accumbens to cause the "high" or euphoric state. However, not all rewards or pleasurable things cause the reward system to be activated which suggests that the mesolimbic system may not just being a system that works merely off things you like (hedonia).[6] Learning on the other hand deals with predictions of future rewards and associative learning. Studies have shown that rats that have a destroyed ventral tegmental area and nucleus accumbens do not lose their learning abilities but rather lack the motivation to work hard for a reward.[5]Incentive salience (wanting) stands out as a possible role of dopamine because it looks at dopamine as being released when there is stimuli worth working hard for, thus making an individual harder to get it. In self-administration studies, animals have been trained to give an operant response (lever press, nose poke, wheel turn, etc.) in order to obtain either a drug or mate. It has been shown that the animals will continue to do the task until the reward is received or fatigue sets in.[2][6]

Since the mesolimbic pathway is shown to be associated with feelings of reward and desire, this pathway is heavily implicated in neurobiological theories of addiction, schizophrenia, and depression.[7][8][9]
Drug addiction, the loss of control over drug use or the compulsive seeking and taking of drugs despite adverse consequences, with the five major classes of abused drugs (psychostimulants,opiates,ethanol,cannabinoids, and nicotine) are due to increased dopamine transmission in the limbic system-each by different mechanisms.[2][10] Like drug addiction, schizophrenia and depression have similar structural changes with dopamine transmission.[7]